37° DISCOVERY REPORTS 



they include some which are of very recent origin and are presumably still in the initial phase of very 

 rapid regression. These probably account for the shoulder in the 'young' frequency curve at 475 cm. 

 The most striking of these frequency distributions is that for ' old ' corpora albicantia, which is very 

 symmetrical and suggests that no more than 30 out of 2339 corpora (1-3%) have either been wrongly 

 allotted to this group instead of to the ' medium ' group, or have disappeared completely. This is the 

 extent of the alteration required to make the curve quite symmetrical, and it may well be the result 

 of chance. The fact that the curve for this group is symmetrical and non-skewed confirms that these 

 corpora represent the final stage of regression in size and that very few are less than 1 cm. in diameter. 

 If 'old' corpora continued to regress in size, but some of those under 1 cm. in diameter were being 

 missed, then the frequency curve should be skewed, with a steeper slope where the lower sizes are 



3 4 



DIAMETER IN CMS. 



Text-fig. 16. Size frequency distribution of 'young', 'medium', and 'old' corpora 

 albicantia. Means + 2 S.E. are indicated. 



cut off. In fact it is symmetrical, like the frequency curve of corpora lutea. This material is con- 

 vincing direct evidence of the persistence of corpora albicantia throughout life in the female fin whale. 

 This conclusion can be checked by two methods which involve only the size of the corpora and not 

 their morphological type. In the first method the size frequency distributions in whales with 1, 2, 

 3, ... to over 30 corpora albicantia have been plotted separately. The modal diameter decreases as 

 the proportion of old corpora albicantia in the samples increases. In Text-fig. 17 the modal diameters 

 have been plotted against the number of corpora albicantia for those groups in which the sample size 

 was over 40. The modal diameter decreases in the first groups from about 4 cm. in the single corpus 

 group to 2 cm. in the groups with over 1 1-12 corpora; the mode then remains more or less the same 

 for the remainder of the groups. If corpora continued to regress in size until they finally disappeared 

 then the modal size should continue to decline instead of stabilizing at 2 cm. This is both the mean and 

 modal size of the ' old ' group of corpora albicantia. 



